Pre-Beta High Density Lipoprotein Unique Disposition of Apolipoprotein A-l Increases Susceptibility to Proteolysis

Apolipoprotein A-l-contalnlng llpoprotelns (high density llpoproteins, HDL) can be separated Into two subtractions, which have pre-beta and alpha electrophoretic mobilities, respectively. These fractions differ In both composition and structure. Some preparations of pre-beta-mlgratlng HDL, but not alpha-migrating HDL, were found to contain two polypeptldes with Mr of approximately 26 and 14 kDa, which are scission products of apolipoprotein (apo) A-l. They are recognized by monospeclflc antibodies to apo A-l and have ^-terminal sequences Identical to those of mature apo A-l. This proteolytic scission of apo A-l occurs primarily after venlpuncture. Immediate addition of protease Inhibitors minimized the appearance of the fragments In plasma. To study the relative susceptibilities of pre-beta and alpha HDL to proteolysis, the llpoprotelns were Incubated In vitro with plasmln. The apo A-l In pre-beta HDL was extensively degraded, but that In alpha-migrating HDL was degraded to a much lesser extent, Indicating that the appearance of apo A-l fragments In pre-beta HDL was due to enhanced sensitivity to proteolysis. To varying degrees, thrombln, kallikreln, elastase, arglnlne C endoprotease, and chymotrypsin also appear to cleave pre-beta HDL faster than alpha HDL Most of the proteases generated a 12 to 14 kDa peptlde fragment under conditions of limited cleavage. These results suggest that the conformatlonal state of apo A-l In pre-beta-mlgratlng HDL or Its spatial relationship to llplds Is significantly different from that of apo A-l In alpha-migrating HDL. Furthermore, this conformation of apo A-l appears to expose a protease-sensltive region near the midpoint of the sequence. Finally, when studies of pre-beta HDL are undertaken, care should be taken to prevent proteolytic degradation of this particle. (Arteriosclerosis 10:25-30, January/February 1990)